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Why is the aquatic nitrogen cycle important to aquaponics? Arguably, the nitrogen cycle is the most important concept to be understood about aquaponics because, without it, there is no aquaponics. The fish eat food and excrete organic waste matter in the form of urea, uric acid arising from the digestion of their food. They also emit ammonia as part of the gaseous exchange that occurs through respiration. The ammonia compounds are toxic to fish and plants cannot absorb ammonia. What would happen if the system was left this way?

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 What allows the fish and plants to survive and thrive?  When ammonia levels in the fish tank reach a certain level, bacteria (Nitrosomonas) begin to colonize the system.  As the bacteria build up, the ammonia is converted to nitrite. As the ammonia levels dip, the nitrite levels increase.  The nitrites (like ammonia) are toxic to fish.  Why is the aquatic nitrogen cycle important to aquaponics?

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 When the nitrite levels reach a certain point, nitrite- oxidizing bacteria (nitrospira) colonize the system and convert the nitrites to nitrates.....which become plant food.  The plants take up the nitrates (and other compounds) and as they grow and are harvested, nitrogen is removed from the system.  Eventually, the system reaches the point where the various aspects of the nitrogen cycle happen simultaneously. As long as the fish are fed (and things remain in balance), the cycle continues.

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Why do plants need Nitrogen? Nitrogen is one of the main elements in protein. Nitrogen is also a component of nucleic acid, DNA, RNA, genes, chromosomes, enzymes, chlorophyll, secondary metabolites (alkaloids), and amino acids. Protein is essential for all living organisms, and is required for growth and development. Nitrogen accounts for about 1 to 6 % of plant dry matter, depending on the species and is often the limiting factor in plant growth.

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Composition of Atmosphere Nitrogen is the major gas in the atmosphere. But it is often the limiting factor in plant growth Why would nitrogen be limiting if it makes up most of the atmosphere?

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Nitrogen Cycle Unlike carbon or oxygen, nitrogen is not very available to life. It’s conversion to a useable form requires biological activity Cyclic conversions from one form to another are mainly mediated by bacteria.

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Cycling of Nitrogen Five processes participate in the cycling of nitrogen through the biosphere: 1) Nitrogen fixation 2) Decay 3) Nitrification 4) Denitrification 5)Dissimilation Microorganisms play major roles in these processes

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 The term nitrification refers to the conversion of ammonium or ammonia to nitrate  Responsible: nitrifying bacteria known as chemoautotrophs.  These bacteria gain energy by converting NH 3 or NH 4 to NO 2 or NO 3  Plants take up NO 3 to make proteins NH 3  NO 2  NO 3 Process 2: Nitrification

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Process 3: Decay  Proteins pass through food webs just as carbohydrates do.  At each trophic level, organic nitrogen compounds are returned to the environment in waste excretions.  Final beneficiaries of these materials are microorganisms of decay.  They breakdown the molecules in excretions and dead organisms into ammonia (NH 3 ). Proteins  NH 3

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 By this process, NO 3 in soil or water is converted into N 2 gas.  This must occur under anaerobic conditions (anaerobic respiration).  Again, mediated by bacteria.  Should not happen in an aquaponics system! Process 4: Denitrification NO 3  N 2

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By this process, nitrates in soil or water is converted back to nitrites and ammonia This must occur under anaerobic conditions. Again, mediated by bacteria. Should not happen in an aquaponics system! Process 5: Dissimilation NO 3  NO 2  NH 3

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Simplified diagram of the nitrogen cycle that is established in an aquatic system

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 Reaction runs best at pH 7-8 and o C.  However; under low Dissolved Oxygen, it runs in reverse.  NO 3 - is converted to NO 2 = and other forms.  Can go all the way backwards to NH 3. Aquatic Nitrogen Cycling

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Ammonia Toxicity Fish excrete ammonia (NH4). When you’re keeping fish at home or in an aquaponics lab it needs to be managed as it is very toxic to the fish. Decomposing food also creates ammonia, so don’t overfeed fish! Some of the effects of excessive ammonia include: – Extensive damage to tissues, especially the gills and kidney – Impaired growth – Decreased resistance to disease – Death Keep below 1 ppm

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Nitrite Toxicity  Now, nitrite is much less poisonous to the fish than ammonia.  But it’s by not good either. It stops the fish from taking up oxygen.  Keep below 5 ppm